This dissertation describes an algorithm and its implementation in the computer algebra system GAP for constructing the cohomology ring and Ext-algebra for certain group algebras kG. We compute in the Morita equivalent basic algebra B of kG and obtain the cohomology ring and Ext-algebra for the group algebra kG up to isomorphism. As this work is from a computational point of view, we consider the cohomology ring and Ext-algebra via projective resolutions.There are two main methods for computing projective resolutions. One method uses linear algebra and the other method uses noncommutative Grobner basis theory. Both methods are implemented in GAP and results in terms of timings are given. To use the noncommutative Grobner basis theory, we have implemented and designed an alternative algorithm to the Buchberger algorithm when given a finite dimensional algebra in terms of a basis consisting of monomials in the generators of the algebra and action of generators on the basis.The group algebras we are mainly concerned with here are for simple groups in characteristic dividing the order of the group. We have computed the Ext-algebra and cohomology ring for a variety of simple groups to a given degree and have thus added many more examples to the few that have thus far been computed.

A concrete description of Hochschild cohomology is the first step toward exploring associative deformations of algebras. In this dissertation, deformation theory, geometry, combinatorics, invariant theory, representation theory, and homological algebra merge in an investigation of Hochschild cohomology of skew group algebras arising from complex reflection groups. Given a linear action of a finite group on a finite dimensional vector space, the skew group algebra under consideration is the semi-direct product of the group with a polynomial ring on the vector space. Each representation of a group defines a different skew group algebra, which may have its own interesting deformations. In this work, we explicitly describe all graded Hecke algebras arising as deformations of the skew group algebra of any finite group acting by the regular representation. We then focus on rank two exceptional complex reflection groups acting by any irreducible representation. We consider in-depth the reflection representation and a nonfaithful rotation representation. Alongside our study of cohomology for the rotation representation, we develop techniques valid for arbitrary finite groups acting by a representation with a central kernel. Additionally, we consider combinatorial questions about reflection length and codimension orderings on complex reflection groups. We give algorithms using character theory to compute reflection length, atoms, and poset relations. Using a mixture of theory, explicit examples, and calculations using the software GAP, we show that Coxeter groups and the infinite family G(m,1,n) are the only irreducible complex reflection groups for which the reflection length and codimension orders coincide. We describe the atoms in the codimension order for the groups G(m,p,n). For arbitrary finite groups, we show that the codimension atoms are contained in the support of every generating set for cohomology, thus yielding information about the degrees of generators for cohomology.

No description available.

The character table of a finite group G is constructed by computing the eigenvectors of matrix equations determined by the centre of the group algebra. The numerical character values are expressed in algebraic form. A variant using a certain sub-algebra of the centre of the group algebra is used to ease problems associated with determining the conjugacy classes of elements of G. The simple group of order 50,232,960 and its subgroups PSL(2,17) and PSL(2,19) are constructed using general techniques. A combination of hand and machine calculation gives the character tables of the known simple groups of order < 106 excepting Sp(4,4) and PSL(2,q). The characters of the non- Abelian 2-groups of order < 2 6 are computed. Miscellaneous computations involving the symmetric group Sn are given.

Made available in DSpace on 2015-03-26T13:45:32Z (GMT). No. of bitstreams: 1 texto completo.pdf: 489385 bytes, checksum: f564b620e335758735ac20b6205a5111 (MD5) Previous issue date: 2010-02-26 / Fundação de Amparo a Pesquisa do Estado de Minas Gerais / In this work, we study the eror-correction codes that are ideals in the group algebra FG(M;N;R) over a field F of characteristic 2, where the underlying group is a non-abelian metacyclic of odd order and has the following presentation: G(M;N;R) = &#8249;a, b : aM = bN = 1, ba = aRb&#8250;; onde mdc(M;R) = 1, RN = 1(mod M) e R &#8800; 1. We use the theory of representations of the metacyclic groups to find the idempotent generators of the minimal central codes of FG(M;N;R) and prove that these codes are combinatorically equivalent to certain abelian codes whose minimum distances are not the best. However, some of these minimal central codes break down into direct sum of minimal left ideals (left codes), which have minimum distances greater than those abelian codes of comparable length and size. Thus, the study of certain metacyclic minimal (left) codes becomes more interesting. A detailed description of the theory of representations of metacyclic groups and some results on group algebras that support the determination of metacyclic codes are initially presented, as well as some results on cyclic codes. / Neste trabalho, estudamos os códigos corretores de erros que são ideais na álgebra de grupo FG(M;N;R) sobre um corpo F de característica 2, onde o grupo subjacente é metacíclico, não abeliano, de ordem ímpar e possui a seguinte apresentação: G(M;N;R) = &#8249;a, b : aM = bN = 1, ba = aRb&#8250;; onde mdc(M;R) = 1, RN = 1(mod M) e R &#8800; 1. Utilizamos a teoria de representações dos grupos metacíclicos para encontrar os idempotentes geradores dos códigos centrais minimais de FG(M;N;R) e provamos que estes códigos são combinatorialmente equivalentes a certos códigos abelianos, cujas distâncias mínimas não são as melhores possíveis. No entanto, alguns destes códigos centrais minimais se decompõem em soma direta de ideais (códigos) minimais à esquerda, que possuem distâncias mínimas maiores que as dos códigos abelianos de comprimento e dimensão comparáveis. Desta maneira, o estudo de certos códigos metacíclicos minimais (à esquerda) se torna mais interessante. Uma descrição detalhada da teoria de representações dos grupos metacíclicos e alguns resultados sobre álgebras de grupo que auxiliam a determinação dos códigos metacíclicos são apresentados preliminarmente, bem como alguns resultados sobre códigos cíclicos.

For a finite group G we study certain rings called k-S-rings, one for each non-negative integer k, where the 1-S-ring is the centralizer ring of G. These rings have the property that the (k+1)-S-ring determines the k-S-ring. We show that the 4-S-ring determines G when G is any group with finite classes. We show that the 3-S-ring determines G for any finite group G, thus giving an answer to a question of Brauer. We show the 2-characters defined by Frobenius and the extended 2-characters of Ken Johnson are characters of representations of the 2-S-ring of G. We find the character table for the 2-S-ring of the dihedral groups of order 2n, n odd, and classify groups with commutative 3-S-ring.

A little over 20 years ago Drinfeld presented the quantum (or Drinfeld) double construction. This construction takes any Hopf algebra and embeds it in a larger quasi-triangular Hopf algebra, which contains an algebraic solution to the constant Yang–Baxter equation. One such class of algebras consists of the Drinfeld doubles of finite groups, which are currently of interest due to their connections with non-Abelian anyons. The smallest non-commutative Drinfeld double of a finite group algebra is the Drinfeld double of D3 , the dihedral group of order six, which was recently used to construct solutions to the Yang–Baxter equation cor- responding to 2-state and 3-state integrable spin chains with periodic boundary conditions. In this thesis we construct R-matrices from the Drinfeld double of dihedral group algebras, D(Dn) and consider their associated integrable systems. The 3-state spin chain from D(D3) is generalised to include open boundaries and it is also shown that there exists a more general R-matrix for this algebra. For general D(Dn) an R-matrix is constructed as a descendant of the zero-field six-vertex model.

A little over 20 years ago Drinfeld presented the quantum (or Drinfeld) double construction. This construction takes any Hopf algebra and embeds it in a larger quasi-triangular Hopf algebra, which contains an algebraic solution to the constant Yang–Baxter equation. One such class of algebras consists of the Drinfeld doubles of finite groups, which are currently of interest due to their connections with non-Abelian anyons. The smallest non-commutative Drinfeld double of a finite group algebra is the Drinfeld double of D3 , the dihedral group of order six, which was recently used to construct solutions to the Yang–Baxter equation cor- responding to 2-state and 3-state integrable spin chains with periodic boundary conditions. In this thesis we construct R-matrices from the Drinfeld double of dihedral group algebras, D(Dn) and consider their associated integrable systems. The 3-state spin chain from D(D3) is generalised to include open boundaries and it is also shown that there exists a more general R-matrix for this algebra. For general D(Dn) an R-matrix is constructed as a descendant of the zero-field six-vertex model.

Made available in DSpace on 2015-05-15T11:46:11Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 411324 bytes, checksum: 65de8bf46cc2dff58911edbcb15868ca (MD5) Previous issue date: 2011-09-09 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work, we study the semisimple group algebras FqCn of the finite abelian groups Cn over a finite field Fq and give conditions so that the number of its simple components is minimal; i.e. equal to the number of simple components of the rational group algebra of the same group. Under such conditions, we compute the set of primitive idempotents of FqCn and from there, we study the abelian codes as minimal ideals of the group algebra, which are generated by the primitive idempotents, computing their dimension and minimum distances. / Neste trabalho, estudamos álgebras de grupos semisimples FqCn de grupos abelianos finitos Cn sobre um corpo finito Fq e as condições para que o número de componentes simples seja mínimo, ou seja igual ao número de componentes simples sobre a álgebra de grupos racionais do mesmo grupo. Sob tais condições, calculamos o conjunto de idempotentes primitivos de FqG e a de partir daí, estudamos os códigos cíclicos como ideais minimais da álgebra de grupo, os quais são gerados pelos idempotentes primitivos, calculando suas dimensões e distâncias mínimas.

Neste trabalho damos uma descrição completa dos ideais à esquerda em anéis de matrizes sobre corpos finitos. Aplicamos estes resultados ao estudo de álgebras de grupo de uma família particular de grupos indecomponíveis e mostramos como construir códigos corretores de erros como ideais destas álgebras. Em particular, exibimos exemplos de códigos tais que, para um dado comprimento e uma dada dimensão, têm o melhor peso possível. / In this work we give a complete description of the left ideals in the full ring of matrices over a finite field. We apply these results to the study of group algebras of a given family of indecomposable groups and show how to construct error correcting codes as ideals of these algebras. In particular, we exhibit examples of codes such that, for a given length and a given dimension, have the best possible weight.